Subwavelength Dielectric Gratings Research Articles

High reflectivity broadband infrared mirrors with all dielectric subwavelength gratings

A high reflectivity (>99%) subwavelength grating mirror (SWGM), yields a very broad reflection spectrum (1.3–2.1μm), has been designed for parallel polarized (p-polarized) incident light and evaluated by rigorous electromagnetic simulations. Design-sensitive analysis was made by varying one parameter at one time while keeping the other constant. The operating region of the grating mirror can be adjusted by varying the parameters of grating. The results show that the broadband reflection is unaffected by the refractive index and the thickness of the low index material, which is different from the previous conclusions. Because of its ease of fabrication, the mirror can be integrated with any photonic component with a flat surface, which has potential applications for active and passive photonic devices.

Improvement of plasmonic field-matter interaction by subwavelength dielectric gratings

We experimentally and theoretically investigate that detection sensitivity in surface plasmon resonance (SPR) biosensors can be significantly enhanced by employing subwavelength dielectric gratings deposited on a gold film. The enhancement originates from an improvement of field-matter interaction: enhanced evanescent field intensity at the binding region and increased surface reaction area. Using a large-area SiO2 grating array fabricated by nanoimprint lithography, experimental sensor performance measured by parylene film coating shows that the SPR substrates combined with a dielectric grating provide a notable sensitivity improvement compared to a conventional bare gold film. We also demonstrate that plasmon field can be more confined and enhanced at the dielectric gratings with a larger width. The proposed SPR structure could potentially be useful in a variety of plasmonic applications including high-sensitivity biosensors.

Fast modal method for crossed grating computation, combining finite formulation of Maxwell equations with polynomial approximated constitutive relations

We present a modal method for the fast analysis of 2D-layered gratings. It combines exact discrete formulations of Maxwell equations in 2D space with polynomial approximations of the constitutive equations, and provides a sparse formulation of the eigenvalue equations. In specific cases, the use of sparse matrices allows us to calculate the electromagnetic response while solving only a small fraction of the eigenmodes. This significantly increases computational speed up to 100×, as shown on numerical examples of both dielectric and metallic subwavelength gratings.

Spectral resonant properties of reflected light for metal dielectric subwavelength gratings in visible regions

A metal-dielectric grating consists of alternating metal and dielectric materials with period less than single wavelength of visible radiations. Optical behaviors of reflection spectra of this grating for s-polarized and p-polarized incident white light are studied systematically. For reflected light, it is the p-polarized light rather than s-polarization shows unusual optical behaviors with characteristics of single-peak spectra, higher peak efficiencies of higher than 75% and lower off-resonant efficiencies. The spectral width of p-polarized light with desirable frequency-selective functions is much wider. There exist two resonant areas for p-polarizations extending toward each other as filling factors increase, and positions of the resonances are mainly determined by grating periods existing linear relationships between them. For making positions of resonances occur in visible wavelengths, filling factors and grating periods should be respectively designed between 0.5 and 0.6 and between 0.25 and 0.45 µm. The newly observed properties of p-polarized lights can be used to exploit novel devices for reflection applications in the fields of optical securities and color filters.

Band structure of plasmons and optical absorption enhancement in graphene on subwavelength dielectric gratings at infrared frequencies

We study theoretically optical properties of a graphene monolayer supported on subwavelength dielectric gratings (SWDGs) using a scattering-matrix method. It is found that due to the formation of band structure of graphene plasmons (GPs) on the SWDG, optical excitations of GPs can be controlled by GP bands above the light line, resulting in optical absorption enhancement in graphene. In particular, very flat GP bands can exist above the light line at midinfrared frequencies. As a result, high optical absorption in graphene can be realized over a wide angular range at these flat bands when using a metallic substrate. Our results could find applications in graphene-based opto-electronics.

Unconsidered aspects of a diffracted field of light through a one-dimensional subwavelength dielectric binary grating

The rigorous coupled-wave technique is implemented to analyze a surface-relief dielectric grating in more and unconsidered details. In this work, the effect of various incident angles upon gratings with different ratio of the period to the incident wavelength were widely investigated for both TE and TM polarizations. Impact of number of space harmonics terms on efficiency of the first and zeroth orders of diffraction were studied for deep and shallow groove gratings. In analyzing the deeper groove grating, it is indicated that more space harmonics are required to approach to a stable solution. In addition, for structures smaller than the incident wavelength it is also shown that the most of the wave are diffracted into the zeroth order of diffraction. But, as soon as the incident angle is increased the first order diffraction efficiency is begin to increase and for larger angles it is being noticeable.

Design of light trapping structures for light-absorption enhancement in thin film solar cells

Light trapping structures with top subwavelength dielectric gratings and back metallic triangle substrate attached to amorphous Si (a-Si) thin film solar cells (TFSCs) are studied. As a result of the grating diffraction effect and surface-plasmon polaritons on substrate, obvious optical absorption enhancement can be achieved in broad wavelength range especially for transverse magnetic (TM) polarized light. Rigorous coupled-wave analysis (RCWA) numerical calculation shows that, compared with bare a-Si TFSCs, the average enhancement is above 20% in the presupposed wavelength range from 0.3 to 1.1μm as the incident angle varies from 0° to 60°, and 30% more absorption would be increased when the incident light is TM only.

Numerical study on an application of subwavelength dielectric gratings for high-sensitivity plasmonic detection

Although subwavelength dielectric gratings can be employed to achieve a high sensitivity of the surface plasmon resonance (SPR) biosensor, the plasmonic interpretation verifying the resulting sensitivity improvement remains unclear. The aim of this study is to elucidate the effects of the grating's geometric parameters on the amplification of SPR responses and to understand the physical mechanisms associated with the enhancement. Our numerical results show that the proposed SPR substrate with a dielectric grating can provide a better sensitivity due to the combined effects of surface reaction area and field distribution at the binding region. An influence of adhesion layer on the sensor performance is also discussed. The obtained results will be promising in high-sensitivity plasmonic biosensing applications.

Design and analysis of multilayered structures with metal–dielectric gratings for reflection resonance and color generation

A multilayered structure incorporating a metal-dielectric subwavelength grating, with the characteristic of polarization separation for visible light and the capability of creating reflection resonance and distinctive polarization-dependent color effects, is proposed. Its reflection resonance and color for TE and TM polarizations are discussed, as well as the influence of its structural parameters on them. Moreover, a reflection filter with characteristics of red for TE-polarized light, green for TM-polarized light, and yellow for daylight is designed and fabricated by replacing its grating region with a sinusoidal grating, and its related characteristics also are verified. The structure and its properties can achieve practical applications in various fields, such as color security, image reproduction, color filtering, and polarization-based information hiding.

Optical properties of sub-wavelength dielectric gratings and their application for surface-enhanced Raman scattering

We fabricated and measured the far-field optical properties of a sub-wavelength Si3N4 (silicon nitride) two dimensional grating. Frequency-dependent transmission measurements from a white-light source revealed that both transverse magnetic (TM) and transverse electric (TE) modes were excited on the grating. We determined the dispersion relations of the modes by tilting the sample with respect to the incoming light beam and measuring the frequency shift of the absorption features. By comparing to a simple model, we determined the effective refractive index for the TM and TE modes and the geometrical constants for the grating. This information enables gratings with desired optical properties to be designed and fabricated. The application of the sub-wavelength grating for surface-enhanced Raman scattering (SERS) is demonstrated.

Discontinuous space variant sub-wavelength structures for generating radially polarized light in visible region

A discontinuous space variant sub-wavelength dielectric grating is designed and fabricated for generating radially polarized light in visible region (λ = 632.8 nm). The design is based on sub-wavelength silicon nitride structures introducing a retardation of π/2 by form birefringence, with space variant orientation of the optical axis. The pattern is divided into concentric ring segments with constant structural parameters, therefore reducing electron-beam writing time significantly. The design avoids the technological challenges encountered in the generation of a continuous space variant grating while maintaining good quality of the resulting polarization mode.

金属膜衬底上亚波长介质光栅结构的特性及传感应用

金属膜衬底上亚波长介质光栅结构的特性及传感应用

High-contrast gratings as a new platform for integrated optoelectronics

A new concept of dielectric subwavelength grating has emerged. This grating leverages a high contrast in refractive indices for the grating medium and its surroundings. We will discuss how high-index-contrast grating (HCG) can manipulate light to achieve various extraordinary properties. We will discuss various designs to yield broadband, high-reflectivity mirrors for light incident in surface-normal direction and at a glancing angle, ultra high-Q resonators with surface-normal output, planar high focusing power reflectors and lenses, and ultralow loss hollow-core waveguides. The HCG will be a new, promising platform for integrated optics with applications for lasers, filters, waveguides, sensors and detectors.

Theoretical analysis of subwavelength high contrast grating reflectors

A simple analytic analysis of the ultra-high reflectivity feature of subwavelength dielectric gratings is developed. The phenomenon of ultra high reflectivity is explained to be a destructive interference effect between the two grating modes. Based on this phenomenon, a design algorithm for broadband grating mirrors is suggested.

Outcoupling surface plasmons into propagation modes using a subwavelength dielectric grating for device applications

Coupling of surface plasmon polaritons to radiation modes by use of a one-dimensional subwavelength dielectric grating on a thin metal slab is discussed. The surface plasmon waves obtained in Kretschmann configuration are resonant outcoupled to radiation modes by using a subwavelength dielectric grating. A peak outcoupling efficiency is predicted to be 74.57% with rigorous coupled-wave analysis. In addition, potential applications of these results in the design and improvement of various optoelectronic devices, such as polarizers, wavelength filters and biochemical sensors are discussed.

Polarization-selective subwavelength grating used with 193 nm light

Hyper-NA ArF (193 nm) immersion lithography is one of the most potential technologies to achieve 32 nm critical dimension node. At the corresponding large angles in the photoresist, control of polarization becomes necessary. A polarization beam splitter (PBS) based on a subwavelength dielectric grating has been designed for use with 193 nm light. The polarization-selective property of such grating is explained by the mechanism of mode interference. The designed grating working as a 1 × 2 beam splitter can transmit TM wave (∼ 90%) to the zeroth order with extinction ratio of 753, and it diffracts TE wave (∼ 80%) to the −1st order with extinction ratio of 300.

Flat dielectric grating reflectors with focusing abilities

Sub-wavelength dielectric gratings (SWG) have emerged recently as a promising alternative to distributed-Bragg-reflection (DBR) dielectric stacks for broadband, high-reflectivity filtering applications. A SWG structure composed of a single dielectric layer with the appropriate patterning can sometimes perform as well as thirty or forty dielectric DBR layers, while providing new functionalities such as polarization control and near-field amplification. In this paper, we introduce a remarkable property of grating mirrors that cannot be realized by their DBR counterpart: we show that a non-periodic patterning of the grating surface can give full control over the phase front of reflected light while maintaining a high reflectivity. This new feature of dielectric gratings could have a substantial impact on a number of applications that depend on low-cost, compact optical components, from laser cavities to CD/DVD read/write heads.

Double negative in flexural single metal layer at visible frequencies

We theoretically investigate a thin single metal layer covered on sub-wavelength dielectric grating. Numerical simulations show that the structure has negative index in the normal direction at visible frequencies. Structural investigation demonstrates that the simultaneous negative permittivity and permeability at an overlapping frequency range are attributed to the metal cut-wire and “U”-shaped structure disassembled from the structure, respectively. Furthermore, parametric exploration for practical fabrication is presented.

Polarization conversion by dielectric subwavelength gratings in conical mounting

Subwavelength dielectric gratings are examined in total-internal-reflection configuration. It is demonstrated experimentally that such elements, fabricated in TiO2, can perform full polarization conversion from incident TE to TM with nearly 100% efficiency. The dependence of the polarization conversion on the angle of incidence is analyzed. Rigorous diffraction theory is used to cross check the experimental results.

Achromatic phase retardation by subwavelength gratings in total internal reflection

Achromatic phase retarders based on subwavelength dielectric diffraction gratings are studied numerically in total-internal-reflection configuration in both the visible and near-infrared regions. Rigorous diffraction theory is used to show that the examined gratings, with reasonable fabrication parameters, can act as quarter-wave plates over a wide range of wavelengths. The best results are obtained if the grating vector is perpendicular to the plane of incidence and the period is slightly below the cutoff period for zero-order operation. Properties of S matrices are used to demonstrate that the main contribution to the broadband effect is not the adjustment of the dispersion of effective refractive indices (like in the previously introduced achromatic diffractive elements), but phase-shift compensation effects caused by the back-boundary reflection.